WASHINGTON (CNN) -- Flying at night in poor weather conditions probably contributed to the crashes of three medical helicopters that killed 11 people in 2007 and 2008, the National Transportation Safety Board said Thursday.

The three crashes occurred near South Padre Island, Texas, in February 2008; Huntsville, Texas, in June 2008; and Whittier, Alaska, in December 2007.

The crashes are among a string of medical helicopter accidents that have raised concerns among NTSB officials.

The NTSB said a fourth accident, in December 2007 in Cherokee, Alabama, was probably caused by the pilot flying too low over trees. The helicopter was shining a searchlight on a hunter who had been lost as rescue personnel on the ground tried to reach him. Three people -- the pilot, a paramedic, and a flight nurse -- were killed, the NTSB said.

The four crashes are among nine medical helicopter crashes in 2007 and 2008 that killed 35 people, according to the NTSB.

The agency Thursday released information on all nine, in addition to the probable causes of the four, and has announced it will hold four days of hearings next month on the issue.

"We have seen an alarming rise in the numbers of EMS accidents," Robert Sumwalt, chairman of the hearing's board of inquiry, said in a written statement.

"This hearing will be extremely important because it can provide an opportunity to learn more about the industry so that possibly we can make further recommendations that can prevent these accidents and save lives," he said.

Sumwalt said the agency recommended to the Federal Aviation Administration in January 2006 that they require all medical operators to develop and implement flight risk evaluation programs, use dispatch and flight procedures that include up-to-date weather information, and install "terrain awareness and warning systems" on their aircraft. A fourth recommendation would require medical flight operators to follow federal regulations regarding their flights.

The NTSB says the recommendations have not been fully implemented.

The recommendations were made after 55 medical aircraft accidents occurred in three years, beginning in January 2002. Some 54 people were killed in the crashes, and 18 people were seriously injured, the NTSB said in a report from 2006.